Referring to FIG. 1, a well known MOS-gated device typically referred to as a trench MOSFET includes a plurality of trenches 10 formed in epitaxial layer 12 which is formed over substrate 14 of a die 8. The vertical sidewalls of trenches 10 are lined with a layer of gate insulation 16 which is typically composed of SiO2. The bottom of each trench also includes a layer of insulation which typically is the same material as the gate insulation material. Each trench 10 is at least partially filled with gate electrode material 18 such as doped polysilicon.
A conventional trench MOSFET also includes base region 20 which is formed in epitaxial layer 12. Base region 20 is doped with dopants of a polarity opposite to those of substrate 14 and those of the rest of epitaxial layer 12. For example, in the device depicted by FIG. 1, base region 20 includes P-type dopants while epitaxial layer 12 and substrate 14 include N-type dopants. A conventional MOSFET also includes source regions 22 formed above base region 20.
Each trench 10 extends to a depth below base region 20 and, therefore, crosses the entire length of base region 20. Upon application of a certain minimum voltage to gate conductive material 18 (threshold voltage) the portion of base region 20 adjacent a layer of gate insulation 16 is inverted to form a channel region which allows a source region 22 adjacent the channel region to be electrically connected to the part of epitaxial layer 12 below base region 20 (the drift region).
A conventional trench MOSFET also includes source contact 24 which is ohmically connected to source regions 22 and highly conductive region 26 formed in base region 20. Source contact 24 is insulated from conductive gate material 18 in each trench by insulation plugs 28. A conventional trench MOSFET such as the one shown by FIG. 1 also includes drain contact 30.
The On-resistance (Rdson) of a trench MOSFET which is the resistance between source contact 24 and drain contact 30 of a trench MOSFET increases with the length of the channel region, which corresponds to the thickness of base region 20.
In order to reduce channel resistance, a shorter channel is desirable. When the channel length is reduced, the trench depth can also be reduced. However, devices having shorter channels tend to be leaky from drain-to-source (Idss). For a device to have the ability to withstand breakdown, it must have sufficient charge in its base regions 20 to support the desired drain-to-source breakdown voltage rating. In devices having shorter channels, therefore, the peak concentration of the dopants in the base region is higher.
The higher peak concentration in the channel leads to a higher threshold voltage, which is undesirable as it increases channel resistance especially at low gate drive conditions. That is, with a conventional short channel device, the peak concentration of the channel has to be made high, which results in a high threshold voltage (Vth).